This invention relates to an aerodynamic shaped article and, more particularly, to such an article with an environmental resistant protective layer on an outer surface.
Various types of dynamic apparatus operate with a flow of fluid to generate power, frequently passing the fluid over an aerodynamic surface. One example of such apparatus is a gas turbine engine in which the fluid is air along with products of combustion resulting from the combustion of fuel. Aerodynamic surfaces in turbine engines include airfoils found in blading members such as rotating and stationary blades, vanes, etc. operating in the flow path of the engine.
Each aerodynamic article, for example the airfoil of a gas turbine engine turbine blade or vane, has an external aerodynamic design shape which is selected in the design of the article to be optimum for the desired fluid flow function of the airfoil. Such function relates not only to the airfoil itself but also to the relationship and cooperation of assemblies of airfoils in a gas turbine engine and the effect such assemblies can have on other parts of the engine. The design of a blading member and its assembly in an engine balances thermal as well as mechanical conditions. Generally an airfoil operating in the strenuous environmental conditions found in the hot sections of a gas turbine engine is air cooled and frequently includes an environmental resistant surface protective layer on the outer surface of the airfoil. The external shape, including thickness, of such an outer layer along with the shape of the underlying airfoil body with which it is bonded is considered in arriving at the original airfoil design external aerodynamic shape for use in an assembly in a particular gas turbine engine.
One example of an outer protective portion or coating commonly applied to an airfoil surface such as a turbine blade or vane is a thermal barrier coating (TBC) system. Such a system generally includes a metallic bond coat bonded with an outer surface of the blade and a non-metallic ceramic type thermal resistant outer layer or TBC bonded with the bonded coat. It has been recognized that it can be beneficial to the thermal balance and to the ultimate operating life of the airfoil of a typical air cooled gas turbine engine blading member if a part of such a protective outer portion or coating system is applied only to that region of the airfoil external surface which has been observed in service operation to experience more strenuous environmental operating conditions than do other regions of the airfoil. For example, it has been observed that a region including primarily the concave surface and leading edge portion of an air cooled airfoil experiences such conditions more than does the convex surface. Therefore, it has been proposed to apply the thermal insulating portion of a protective outer coating system, for example the TBC, only to the region generally including the concave surface and leading edge portion, leaving the balance of the airfoil with the bond coat or other metallic environmental resistant coating.
Such a change only in external coating distribution on an existing blade aerodynamic design without change in the combination of the shape of the airfoil body and the coating would change the external shape of the airfoil from the design external shape to one less than optimum. Change in such shape from design affects airflow characteristics and loading on the blading members and can transfer adverse loading conditions to associated engine members, for example bearings.